v6.5 fixed almost twice as many "high" CVEs (19) than the second most prolific release, v6.6 (11), with v6.4 tied for 3rd place (9) with v5.17. It seems like the rate of fixing is picking up.
Ignoring the first git release (v2.6.12), the "high" flaw counts are relatively even. The most flawed (i.e. most well tested/researched) releases have been v3.8 (9), v3.18 & v2.6.20 tied (8), and v5.9 & v4.1 tied (6).
But there are certainly more flaws in all releases -- they just haven't been found yet.